In systems which store quantities of liquid samples, e.g. blood samples, chemical samples or other biological samples, robotically operated automated storage systems are commonly used and these can store many thousands or even millions of individual samples in small tubes. Access to the individual tubes is required periodically and it is useful for the operator of such a system to have an indication of the amount of sample stored in a particular tube so that as quantities of the sample are removed over time, an ongoing check can be kept on the volume still available.
Individual sample tubes are typically configured to have a maximum volume of a few milliliters, with typical volumes being 0.3 ml, 0.75 ml, 1.4 ml and 2 ml. A given quantity of tubes is normally stored in a rack which can hold a certain quantity of tubes, e.g. 96 tubes. One storage system can comprise a variety of different tube sizes.
A prior art solution to the problem of assessing volume involves the manual inspection of a particular tube to assess the volume remaining, and this may be supplemented by an estimate from a user. Obviously such a solution is very labour intensive and may not be used when a great quantity of tubes requires verification.
Other, more automated, solutions exist. One of these involves the accurate weighing of a tube, which can give the weight of the sample once the nominal weight of the empty tube is subtracted therefrom. However, this can be a time consuming task and requires individual tubes to be assessed separately.
There are devices available which aim to expedite this process. One such device processes a rack of tubes by selecting a particular tube, reading its identifying bar code and then weighing it. The resulting data is then stored in a file which can be reconciled with the inventory of the entire stock of tubes. Such devices can also be used to pre-weigh the tubes so that the later weight calculation is relatively easy to do. However, this further complicates the inventory system. Also, such devices tend to be quite slow in operation and can take between 20 and 30 minutes to individually weigh a rack consisting of 96 tubes. Furthermore, if individual tubes are not pre-weighed there is a question about how accurate the subsequent volume estimating can be given that there is a noticeable difference in the weight of individual tubes and it has been seen that these can vary by as much as 20 mg.
An alternative approach to using weight to infer a volume in a tube is to utilize a non-contact liquid level detection. This approach uses one or more sensors which are operable to determine the distance between the sensor and the surface of the liquid in a tube. By use of a suitably known tube, the level of the liquid may be used to determine the volume of sample in the tube. An advantage of such a sensor is that it is able to operate at a higher speed than the weighing solution discussed previously. However, a particular shortcoming of such a device is that the tube cap or septa must be removed in order for the upper level of the liquid to be exposed. In addition to increasing the risk of sample cross contamination, this step has major implications for sample quality, unless it is performed in a controlled environment, e.g. a low humidity environment, to prevent the uptake of moisture, which could, of course, upset the volume calculations.